The present invention relates to contrast sensitivity testers and, more particularly, to contrast sensitivity testers using projected images produced by vectographic slides or vectographic near-point cards.
Vectographic slides have been used for many years to test for monocular acuity under binocular conditions, axis of astigmatism of each eye under binocular conditions, acuity and accommodative balance between the right and left eyes, binocular instability, parity between monocular and binocular acuity, muscle balance, fixation disparity, and stereo acuity. One early report on the use of such slides was published in the New England Journal of Optometry, May 1966, Vol. XVII, No. 5. The unique feature of these slides is the presence of polarized symbols against an unpolarized background portion; typically, some symbols are polarized in one direction and other symbols are polarized in the orthogonal direction. The traditional tests noted above are conducted using a pair of orthogonal polarization analyzers, one in front of each eye, to make each symbol visible to one eye and invisible to the other eye. Positioning of the analyzers at exactly 90.degree. is not required, since unwanted ghost images do not appear until the analyzers are fully 20.degree. out of alignment. Most subjective refractors are offered to practitioners with orthogonal analyzers mounted in the accessory wheels.
Since most practitioners already use vectographic slides and near-point cards, the adaptation of a refractor to provide rotatable analyzers instead of, or in addition to, the fixed analyzers offers a simple means for contrast sensitivity testing in addition to the conventional vectographic tests. Alternatively, a rotatable analyzer can be added to a chart projector. Vectographic slides and vectographic near-point cards, which may include slides and near-point cards having test symbols polarized in one direction only, may be used in conjunction with rotatable analyzers for testing contrast sensitivity. One advantage of a refractor containing rotatable analyzers is that contrast sensitivity under reading conditions can be tested using vectographic near-point reading cards. Vectographic near-point reading cards for use in binocular refraction, and a process for making them, are disclosed in U.S. Pat. No. 3,572,911 issued Mar. 30, 1971; it will be understood by those skilled in the art that the disclosed process may be easily modified to produce near-point cards having a symbol or symbols polarized in one direction only.
Heretofore, contrast sensitivity testing has been conducted using special charts containing symbols having different contrast levels relative to the chart background or slides that project symbols of different contrast levels. Such devices have the inherent disadvantage that there is necessarily a predetermined finite step between each contrast level, whereby the precision with which contrast sensitivity may be determined is limited.
Thus, it is an object of the present invention to provide an improved means of testing contrast sensitivity whereby the relative contrast of a test symbol in relation to a reference background may be continuously, rather than discretely, varied by the practitioner.
It is a further object of the present invention to provide a means of testing contrast sensitivity using vectographic slides or vectographic near-point cards rather than specialized contrast-level type charts or slides.
Another object is to enable practitioners to conduct an improved test for contrast sensitivity through simple modification of a conventional refractor or chart projector.
Other objects, advantages, and features of the present invention will become apparent from the following description and taken in connection with the accompanying figures.